Ooops again!
Yes - I did know that, but I'd forgotten that when after reading the posts - I just didn't want people thinking we were trying to log something we hadn't seen - my original question could be read in two ways, and some of the other posters thought that we wanted to log items that had gone through our cache but we hadn't seen them.
To go back to my original point, from the replies, we're OK to visit our cache to either discover, or collect any trackables if we can help them on their mission - so all sorted!!
Thanks all for the fast answers!